alt Hacker NewsSame with the VIC-II and the 6510 in the Commodore 64. The video chip is given the main character role for the bus, stopping the CPU from moving forward if it needs cycles for video generation or DRAM refresh.
Same with the VIC-II and the 6510 in the Commodore 64. The video chip is given the main character role for the bus, stopping the CPU from moving forward if it needs cycles for video generation or DRAM refresh.
The clever thing about the Apple II is that there are no refresh cycles. Woz laid out the screen buffer in memory in such a way that simply scanning out the screen will touch every single row of DRAM.
This is more about saving chips than saving cycles, since the Apple II was implemented entirely with 74 series logic. A more traditional approach that used spare cycles during horizontal blanking would have required several more chips.
It does mean that the layout of the Apple II's screen memory is somewhat insane. Those DRAM chips needed to be refreshed every 2ms, and it takes 16ms to scan out a whole screen. Every 8th of the screen needs to be spread out across all 128 rows.